One of the major contributing factors to the deterioration of specific fuel consumption of aircraft gas turbine engines is the gradual increase in the clearance between the rotor blade tips and the surrounding, nonrotating annular seal ring. As will be appreciated by those skilled in the art, increased clearance between the rotating and stationary components increases the leakage of the working fluid of the engine across individual rotor stages in either the compressor or turbine sections. Such leakage reduces overall engine efficiency hence raising the total specific fuel consumption.
This increase is directly related to the accumulated usage of an engine since the most recent seal installation and/or replacement. The blade tip clearance, and hence sealing effectiveness, are at a minimum immediately following installation, and deteriorate as the engine experiences extended running time and/or repeated use cycles. Eventually the deterioration of the blade tip clearance reaches a sufficient level so as to economically warrant replacement of the worn components, a time consuming and expensive task.
Methods of improving both the wear life and effectiveness of blade tip seals have long been sought by engine designers. One effective method, termed "active clearance control" recognizes that one principal source of seal wear is the differential thermal growth of the rotor and surrounding engine casing which supports the annular seal ring. Active clearance control, as disclosed in U.S. Pat. No. 4,069,662, uses a modulated flow of cool air derived from the engine bypass airflow stream and exhausted adjacent the high pressure turbine casing to shrink the casing relative to the high pressure turbine blade tips under steady state, high altitude cruise conditions. By providing such a modulated flow of cooling air at preselected engine operating conditions, active clearance control allows the engine to operate with minimum seal clearance for the majority of its operating cycle while reducing or eliminating the interference or abrasion between the seal ring and blade tips which can occur during transient conditions such as takeoff, throttle back, etc.
Although a major advance in the art of seal effectiveness and efficient gas turbine engine operation, engines equipped with active clearance control are still subject to accumulated, engine usage related blade tip clearance deterioration. It is known to attempt to measure the current blade tip clearance in an operating engine for the purpose of modifying such clearance by either mechanical or thermal means, however such measuring methods have not proved as accurate and dependable as would be required in an operating aircraft environment.
What is required is a method for maintaining or restoring optimum blade tip clearance in an aircraft gas turbine engine between scheduled periodic seal replacement.